The present invention relates to a fuel cell with improved separation. It relates in particular to a fuel cell which is essentially constituted by a plurality of identical juxtaposed elements which are disposed in electrical contact with one another.
Overall, such a cell structure includes three distribution circuits which are common to all of the cells:
an electrolyte distribution circuit for distributing electrolyte from an external source and including, in particular, a common inlet channel and a common outlet channel;
a fuel distribution circuit for distributing fuel from an external source and including, in particular, at least one fuel inlet channel and at least one outlet channel for removing unburnt fuel and inert gases from the cell structure; and
an oxidant distribution circuit including, in particular, a plurality of oxidant inlet and outlet orifices situated in the bottom and top faces of the cell structure.
In addition, each cell comprises, in outline:
first and second porous electrodes which are preferably plane in shape with parallel faces, one being a cathode and the other an anode, and each including a specific catalyst;
an electrolyte filling the gap situated between the said electrodes; and
first and second impermeable bipolar current collectors comprising respective first and second frames of plastic material having at least one central conductive zone having channels on each of its faces, the first collector coming into electrical contact via the high points of its cathode face with the external surface of the said cathode, and via its anode face with the external surface of the anode of an adjacent cell, and the second collector coming into electrical contact via the high points of its anode face with the external surface of the said anode, and via its cathode face with the cathode of the other adjacent cell; the oxidizing gas from the said common oxidant distribution circuit flowing between the said cathode and the cathode face of the first collector and being supplied to the said cathode, and the fuel gas from the said common fuel distribution circuit flowing between the said anode and the anode face of the second collector and being supplied to the said anode; the first and second frames being provided with orifices to allow the electrolyte and the fuel to flow through the cell structure, the said orifices contributing by their juxtaposition to defining the said common inlet and outlet channels. Means are provided on the anode face of the second collector to enable the fuel from the inlet channels to be conveyed over the surface of the anode and away from said surface to the outlet channels; said means including grooves constituting a plurality of microchannels, which grooves are disposed in the top and bottom portions of the said second frame.
Further, the first electrode is applied against a third frame made of plastic material and having a central orifice in the form of a quadrilateral, the top and bottom portions of the third frame including orifices for conveying fuel and electrolyte through the cell structure, said orifices corresponding to the orifices for conveying fuel and electrolyte through the cell structure, said orifices corresponding to the orifices provided in the frames of the collectors, and likewise contributing to the formation of the said inlet and outlet channels for the fuel and the electrolyte; means being provided on one face of the third frame to enable electrolyte to be conveyed from the inlet channel to an electrolyte compartment situated between the two electrodes, and from the said compartment to an outlet channel; the said means including grooves that open out into set back portions that constitute electrolyte distributing and collecting manifolds disposed on the top and bottom portions of the third frame along two opposite sides of the central orifice and in communication with the electrolyte chamber by means of a plurality of parallel microchannels delimited by ribs.
The second electrode is applied against one of the faces of a fourth frame facing the said current collector; the fourth frame is made of an insulating plastic material, is of identical contour to the third frme against which the first electrode is applied, and has fuel and electrolyte conveying orifices corresponding to those provided in the said third frame. The fourth frame is applied by a plane face against the face of the said third frame having the grooves and the electrolyte distributing and collecting manifolds.
When a repetitive stack is made using a predetermined elementary sequence of frames as defined above having a series of superposed orifices, a cell structure is obtained having a plurality of cells connected electrically in series together with a common channel for conveying electrolyte into each of the electrolyte compartments, a common cahnnel for removing the electrolyte, a plurality of common channels for supplying all the anodes with fuel, and a plurality of common channels for evacuating gases.
The oxidant flows through the stacked cell structure by passing directly through the cathode compartments in a vertical direction via orifices and distribution means provided at the bottom and top edges of the corresponding frames.
All these plastic material frames are thin, they may be between 0.2 mm and 5 mm thick; and advantageously they are molded.
The frame material may be selected from synthetic insulating materials of the following types: polypropylene, polyethylene, polyvinyl chloride (PVC), acrylonitrilebutadiene-styrene (ABS), polysulfone, polystyrene.
It is known that each electrolyte compartment may contain a microporous separator constituted, for example, by a corrugated sheet. This electrolyte-impregnated separator presents a degree of resistance to the passage of ions, thereby leading to a small, but non-negligible, ohmic drop.
A separator may have one or more of the following functions.
It should be capable of maintaining a certain distance between the electrodes and thus avoiding any possibility of a short circuit, regardless of the cause bringing the electrodes together, e.g. the collector-electrode assemblies being warped, local electrode unsticking, etc . . .
It must preferably be capable of maintaining a constant gap between the electrodes over the entire surface of an element so as to ensure uniformn electrolyte flow.
It must constitute a wall or a hydrophilic network which facilittes complete filling of the electrolyte compartment with electrolyte solution by the wick effect, which filling is made difficult by the hydrophobic or semi-hydrophobic nature of the two walls which constitute the surfaces of the two electrodes.
It must make it possible, by virtue of its hydrophilic surface, to establish a continuous or quasi-continuous layer of liquid which is capable of remaining intact even if the electrolyte compartment is accidentally emptied, e.g. by an electrode being perforated. This layer of liquid thus constitutes a screen preventing direct mixing of the two gases and the possible consequences of such mixing such as ignition and combustion inside the cell, explosiuon, etc . . .